The long-term objective of this proposal is to understand the molecular mechanisms underlying the biological responses to dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD) exposure. TCDD, the prototypic dioxin and a model for many other organochlorinated compounds, produces many apparently unrelated biological effects, ranging from chloracne in humans to developmental teratogenesis, tumor promotion, thymic atrophy, wasting syndrome and death in laboratory animals. In addition, TCDD, a rodent carcinogen, is strongly suspected of being carcinogenic also in humans. The molecular basis of the biological effects of TCDD is largely unknown. Dioxin is a ligand for the aromatic hydrocarbon (Ah) receptor (AHR), which, as a dimer with the Ah receptor nuclear translocator protein ARNT, mediates the transcriptional activation of genes in the CYP 1 family of cytochrome P450 monooxygenases. However, activation of the CYP1A1, CYP1A2 and CYP1 B1 genes, although one of the best characterized effects of Ah receptor activation by TCDD, does not adequately explain the diversity of TCDD effects. Our recent global expression profiling analyses of human hepatoma cells shows that exposure to dioxin induces or represses a total of more than 300 genes, with repression being the more frequent. Induction may readily be explained by the transactivating potential of the AHR, but gene repression is a novel effect of the activated AHR that is uncharacterized at the molecular level. The goal of the experiments proposed here is to define and characterize the regulatory interactions between the activated AHR and other transcription factors, co-regulators and chromatin remodeling factors responsible for the effects of dioxin on gene expression. The major objectives of this work are, (1) to define the role of discrete domains of the AHR in gene regulation; (2) to use proteomic analyses to identify AHR coregulatory partners in gene induction and repression; and (3) to clone the promoters of AHR regulated genes and characterize their response to dioxin exposure. Results from these experiments will be crucial for our understanding of the long-range biological consequences of exposure to dioxin and to other organochlorinated compounds and will help formulate an adequate rationale to deal with health problems arising from an ever-increasing exposure to these environmental agents.